PROJECT SUMMARY/ABSTRACT Osteoporosis is a common, chronic disease with an enormous health burden. There is an urgent need for new anabolic therapies. The long-term goal of this project is to understand genetic risk factors for osteoporosis to identify new drug targets to reduce this massive health burden. Genome wide association studies (GWAS) have identified hundreds of loci associated with bone mineral density (BMD). However, the causal genes at these loci remain to be discovered. The rationale for this project is that defining causal genes at BMD loci is, at present, a phenotype-limited problem. More specifically, most causal genes reside in the genomic regions flanking BMD loci. Thus, each locus implicates multiple candidate genes residing in these flanking regions. However, there are 100s of such candidate genes whose skeletal functions are unknown, and in vivo approaches with sufficient throughput to fill this phenotype gap are virtually non-existent. There is an urgent scientific need to understand the functions of genes at BMD loci, because they form the basis for understanding how genetic variants influence BMD, and in turn, the ability to translate these loci into clinical targets. The objective of this project is to leverage rapid-throughput biology in zebrafish to advance our understanding of genes at BMD loci that influence bone mass and quality, and the mechanisms by which they act. Our central hypothesis is that genetic variants influence BMD by regulating genes at BMD-associated loci, which work in concert to influence bone mass and quality. In specific aim 1 (SA1), we will identify genes at BMD loci with adult loss-of-function skeletal phenotypes in a reverse genetic screen. Our team has identified 56 BMD loci in a large-scale GWAS meta-analysis. A novel phenotyping strategy will be employed to functionally annotate candidate genes residing within 80kb of these 56 BMD loci. Human genetic analyses will be performed to explore how each gene identified in our screen contributes to BMD. In specific aim 2 (SA2), we will perform a multi-level examination to determine cellular and molecular mechanisms by which genes at 7q31.31 influence bone mass and quality. SA2 will serve as a model by which new genes discovered in SA1, and their interactions with each other, will be mechanistically evaluated. This project is innovative because it substantially differs from the status quo for functional testing for causal genes at BMD loci?the use of mouse phenotyping consortiums?and harnesses approaches developed by our team to perform one of the most comprehensive functional analyses of genes at BMD loci to date. This project is significant because it will: 1) establish an efficient model for exploration of human genomics underlying osteoporosis-related traits, for which there is an urgent need; 2) identify new skeletal genes at BMD loci, a genetic territory that is enriched with known osteoporosis drug targets, and which has proven to yield drug targets likely to be clinically viable; 3) define mechanisms by which genes at 7q31.31 influence bone mass and quality, as a model of how different genetic variants and their causal genes can act in concert to affect BMD.